The present invention relates generally to electromagnetically reading coordinate data to obtain digital equivalents of the data and to a digitizer system having such a function.
A typical electromagnetic digitizer known heretofore comprises an exciting coil for producing an alternating magnetic field and two mutually perpendicular sets of parallel conductors arrayed like a reticulate lattice. The magnetic field induces electromotive forces in the conductors. The exciting coil (hereinafter shall be referred to as a cursor coil or simply as a coil) is housed in a cursor or stylus capsule of the digitizer movable along the surface of the lattice. The conductors in one set are electrically insulated from the other set of conductors. These conductors are embedded in a planar substrate to form a so-called tablet. The conductors of each set have their one ends electrically connected together so that m longitudinal loops X.sub.i (i=1.about.m) and n lateral loops Y.sub.j (j=1.about.n) are formed by the respective adjacent conductors, as shown in FIG. 1.
To determine the position of the cursor, alternating voltage of tens Hz to several MHz is applied across the coil C housed therein to induce alternating electromotive forces in the respective loops X.sub.i and Y.sub.j. The electromotive force is measured by sequentially changing over the switches S.sub.x and S.sub.y from the loops X.sub.1 and Y.sub.1 through to X.sub.m and X.sub.n, respectively. Then, reading the address or coordinates (x.sub.i, y.sub.j) of a pair of one X.sub.i and Y.sub.i under the highest electromotive force in each set of loops gives the position of the cursor.
In such a method, however, the positional resolution depends on the intervals between adjacent loops or conductors. Therefore, to improve the resolution, the conductors must be arrayed at correspondingly decreased intervals, or positions between adjacent conductors must be determined somehow. However, it is difficult to make the intervals narrower than about 0.5 mm in view of the manufacturing technique and cost or from a quality control aspect. Therefore, if a resolution higher than this level is required, it is necessary to determine such inter-conductor positions by some means or other. Hereinbelow, a few known solutions for determining such inter-conductor positions are described with reference to FIGS. 2, 3 and 4.
FIG. 2 shows typically how the peak value of alternating electromotive force induced in a loop L by a coil C changes as the coil moves transversely of the loop L. The diameter d of the coil C is almost equal to the interval between two conductors forming the loop L.
To determine the position of the coil in a loop, namely, its position between two adjacent conductors forming the loop, one known method utilizes such a change in the induced electromotive force with the position of the coil relative to the loop. This method uses a tablet having its alternate conductors connected together at their one ends so that the resultant one set of loops formed of adjacent odd conductors are half-lapped over the other set of loops formed of adjacent even conductors, as shown in FIG. 3(a). If the interval between two conductors forming one loop is almost equal to the diameter of the coil C like the case of FIG. 2, the electromotive forces e.sub.i and e.sub.i+1 induced in the loops L.sub.i and L.sub.i+1 will have waveforms as shown in FIG. 3(b) with their peaks corresponding to e.sub.a, e.sub.b or e.sub.c shown in FIG. 2, when the coil C occupies the respective positions relative to the loops L.sub.i and L.sub.i+1 as shown in FIG. 3(a). Then, the phase of e.sub. i+1 is advanced by 90.degree. to obtain the waveform e.sub.i+1, which is in turn added to e.sub.i to obtain e.sub.p. The resultant waveform e.sub.p has a phase difference from the original e.sub.i or e.sub.i+1 which varies with the position of the coil C relative to the loops L.sub.i and L.sub.i+1. Thus, the position of the coil C can be known by determining this phase difference of e.sub.p from e.sub.i or e.sub.i+1. With this method, however, it is difficult to achieve a high accuracy. Further, rather intricate operations are involved such as phase shift, superposition of waves and measurement of phase difference.
Alternatively, Japanese Patent Provisional Publication No. 52-96825 discloses the use of a coil having a diameter d about three times larger than the interval l between adjacent conductors as shown in FIG. 4. In this arrangement, the inter-conductor position is determined based on the finding that the ratio e.sub.m /e.sub.s of the highest electromotive force e.sub.m to the second highest electromotive force e.sub.s among the three loops substantially covered by the coil C is a function of its inter-conductor position. However, since this function is not a linear one, complicated calculation is required, or the positional reading must be compensated by using data precollected and stored in a ROM or the like.